Endoscopic work to be performed intra-abdominally, in particular on the internal genitalia or other parts, is carried out with the use of laser light transmitting fibers, as described, for example, in Federal German Patent Specifications Nos. 28 52 653 and 29 45 080, with the aid of fiber guiding instruments. In such fiber guiding instruments, the fiber is angularly deflected at the distal end of the instrument towards the treatment site, the radius of curvature of such deflection commonly being very small, so that the somewhat brittle fiber is susceptible to breakage. Also, in the known instruments, a fiber deflecting fiber guide pivoted to said distal end is operated by means of a handle which is pivotable about the longitudinal axis of the instrument at the proximal end thereof. Unless the instrument is locally immobilised when the handle is being operated, accidental rotary movement of the distal end of the instrument will occur, whereby the distal end of the fiber is swung out of the treatment site. Further, in the case of these known instruments, the pivotal movement of said fiber guide at the distal end of the instrument and the rotary movement of the handle at the proximal end are contradirectional so that skill and experience in using the instrument are needed if faulty work is to be avoided.